1. Field of the Invention
The present invention generally relates to an active noise control scheme for reducing aircraft engine noise and, more particularly, to a noise control system incorporating compact sound sources and distributed inlet error sensors for reducing the noise which emanates from an aircraft engine inlet of a gas turbine engine.
2. Description of the Prior Art
Noise has been a significant negative factor associated with the commercial airline industry since the introduction of the aircraft gas turbine engine. Considerable effort has been directed toward quieting aircraft engines. Much of the progress to date is associated with the development of the high bypass ratio turbofan engine. Because the jet velocity in a high bypass engine is much lower than in low or zero bypass engines, the exhaust noise associated with this engine is greatly reduced. Although exhaust noise in high bypass engines has been greatly reduced, fan and compressor noise radiating from the engine inlet remains a problem. In fact, as turbine engines evolved from turbojet to primarily turbofan engines, fan noise has become an increasingly large contributor of total engine noise. For high bypass ratio engines (i.e. , bypass ratios of 5 or 6) currently in use, fan noise dominates the total noise on approach and on takeoff. More specifically, the fan inlet noise dominates on approach, and the fan exhaust noise on takeoff. However, acoustic wall treatment has only made small reductions in fan inlet noise levels of less than 5 dB. This is compounded by inlet length-to-radius ratio becoming smaller. A typical fan acoustic spectrum includes a broadband noise level and tones at the blade passage frequency and its harmonics. These tones are usually 10 to 15 dB above the broadband level. This is for the case where the fan tip speed is subsonic. Multiple pure tones appear as the tip speed becomes supersonic.
Not only is fan noise a problem in existing aircraft engines, it has been identified as a major technical concern in the development of the next-generation engines. Rising fuel costs have created interest in more fuel-efficient aircraft engines. Two such engines currently in development are the advanced turbo-prop (ATP) and the ultra-high-bypass (UHB) engines. Although attractive from the standpoint of fuel efficiency, a major drawback of these engines is the high noise levels associated with them. Not only will the introduction of ultra high bypass ratio engines in the future, with the bypass ratios in the range of 10, result in a greater fan noise component, with shorter inlet ducts relative to the size of the fan and for the lower blade passage frequencies expected for these engines, passive acoustic liners will have greater difficulty contributing to fan noise attenuation because liners are less effective as the frequencies decrease and the acoustic wavelength increases. Because of these difficulties, it is likely that passive fan noise control techniques, while continuing to progress, will be combined with active noise control techniques to produce a total noise control solution for fans.
For subsonic tip speed fans, noise is produced by the interaction of the unsteady flows and solid surfaces. This could be inflow disturbances and the inlet boundary layer interacting with the rotor or the rotor wakes interacting with the stator vanes. Acoustic mode coupling and propagation in the duct and, in turn, acoustic coupling to the far field determines the net far field acoustic directivity pattern.
Reduction of noise from the fan of a turbomachine can be achieved by reduction of the production processes at the source of the noise or by attenuation of the noise once it has been produced. Source reduction centers on reduction of the incident aerodynamic unsteadiness or the resulting blade response and unsteady lift or the mode generation and propagation from such interactions.
Most efforts at noise reduction in this area are passive in nature in that the reduction method is fixed. Examples include the effects of respacing the rotor and stator or the spacing of the rotor and downstream struts. However, there have been some efforts at active control of these source mechanisms. Preliminary experiments have shown the attenuation of noise from an incident gust on an airfoil by actuating a trailing edge flap to control the unsteady lift. In general, an attempt to alter source mechanisms will require engine redesign and the effect on performance will have to be assessed.
Efforts to date at reductions in source noise have been insufficient in reducing overall engine noise levels to the required levels. The additional reductions have been met with passive engine duct liners. The contribution of duct liners is primarily in attenuating fan exhaust noise where the propagating modes have a higher order and propagate away from the engine axis where liners can be most effective. In the fan inlet, the modes are propagating against the boundary layer and are refracted toward the engine axis, minimizing the effectiveness of liners.
Another option for turbofan noise reduction is to actively control the disturbance noise with a second control noise field. The concept of active sound control, or anti-noise as it is sometimes referred to, is attributed to Paul Leug. See U.S. Pat. No. 2,043,416 to Leug for "Process for Silencing Sound Oscillations". The principle behind active control of noise is the use of a second control noise field, created with multiple sources, to destructively interfere with the disturbance noise. A further distinction can be made if the control is adaptive; that is, it can maintain control by self-adapting to an unsteady disturbance or changes in the system.
While Leug's patent is almost sixty years old, only in the past ten to twenty years has active control begun to converge in many applications. The applications of active control were made possible by the advancements in digital signal processing and in the development of adaptive control algorithms such as the very popular least-mean-square (LMS) algorithm.